The present invention relates to a thread rolling die and a process for the production thereof.
Formation of grooves in, e.g., ball screw shaft can be accomplished by rolling or grinding. The former process is used to produce a general-purpose ball screw, while the latter process is used to produce a precision ball screw. The formation of grooves for rolled ball screw can be accomplished by a process of transferring shape of over two or three roll dies (hereinafter referred to as xe2x80x9crolling diexe2x80x9d). Thus the formation of grooves by the process of transferring gives a higher productivity than grinding. The rolling process enables mass production at a reduced cost according to standard specification.
An example of the conventional die is disclosed in JP-A-9-133195. As shown in FIG. 6, a rolling die 1 comprises a cylindrical portion 2, and conical portions 3, 3xe2x80x2 which are connected with the cylindrical portion 2 at both ends thereof and diameter of which each become smaller towards the respective outer end thereof. A helical protrusion (dancette portion) for rolling 8 is continuously formed over the area ranging from one conical portion 3 to the other conical portion 3xe2x80x2 through the cylindrical portion 2. A face connecting the crowning of the helical protrusion 8 on the cylindrical portion 2 forms a cylindrical side face 6 along the periphery of the cylindrical portion so as to give a straight portion S. Faces connecting the crowning of the helical protrusion on the conical portions 3, 3xe2x80x2, each form conical side faces 7, 7xe2x80x2 along the inclined surface of the conical portions. The conical side surface 7 forms a predetermined angle xcex8 (from 220  to 15xc2x0) of inclination with respect to the cylindrical side face 6 in order to form a biting portion K. On the other hand, the conical side surface 7xe2x80x2 forms a predetermined angle xcex8xe2x80x2 (from 2xc2x0 to 90xc2x0) of inclination with respect to the cylindrical side face 6 to form a relief portion N.
In the case where this conventional rolling die is used to form a thread in a ball screw shaft by rolling, the lead angle of the rolling die 1 and a lead angle of a rod material are each deviated by a predetermined value, when the rolling die 1 is pressed against the rod material. In this manner, the rolling die 1 and the rod material come into relative rotation as well as walking phenomenon. As a result, the rolling die 1 and the rod material automatically come into rolling while making relative movement in their axial direction (longitudinal direction). During rolling, the rolling die 1 relative moves toward the side of its biting portion K. Since a threaded groove is formed in the rod material with respect to gradually increasing of the amount of biting by the rolling die 1 which is according to the magnitude of the angle xcex8 of inclination, the resistance during rolling can be lowered.
The process, which comprises forming thread while giving a longitudinal feed by the walking phenomenon developed by deviation of the lead angle of the rolling die and the rod material, is referred to as xe2x80x9cthrough-rollingxe2x80x9d, and this process has heretofore been widely practiced.
Thus, in the conventional rolling die, longitudinally sectional shapes of helical protrusions (dancette portion) on the tapered biting portion K and relief portion N are different from that of the untapered straight portion S. In order to produce the conventional rolling die by grinding, different grinding wheels must be used to grind the straight portion S, and to grind the biting portion K and the relief portion N. FIG. 7 shows an example of the conventional process for the production of a rolling die. A grinding wheel A is used to grind the straight portion S of the rolling die 1. Another grinding wheel B is used to grind the biting portion K. A further grinding wheel C is used to grind the relief portion N, which is oblique in the direction opposite the biting portion K. Thus, the grinding wheels are exchanged at every portion. Grinding is independently effected while the axial position being controlled.
In the case of the conventional rolling die, however, the dancette portions on the biting portion K, the straight portion S and the relief portion N formed by different grinding wheels can easily have different shapes. In addition the connection between these portions difficulty have desired precision in working. Therefore, the conventional rolling die is disadvantageous in that (i) the connection between the various portions has an edge and (ii) the use of a plurality of grinding wheels A, B and C adds to cost. Further, every time the angle xcex8 of inclination of the biting portion K and the angle xcex8xe2x80x2 of inclination of the relief portion N differ from each other, another grinding wheel must be accordingly prepared. Thus, it is not practical from the standpoint of cost and precision to provide the biting portion K with a plurality of different angles of inclination. Moreover, it is impossible to provide the biting portion K with a continuous change of these angles of inclinations.
In recent years, there has been growing a demand for a rolled-ball screw which can be produced at a high productivity but has the same performance as in the precision ball screw produced by grinding. However, it is likely that the ball screw, which is produced by rolling with the use of a conventional rolling die having a poor precision in transfer of dancette portion and in connection between the various portions, cannot meet severe requirements such as improvement of precision in positioning, improvement of life and reduction of noise.
Further, when a conventional rolling die having an edge on the connection between various portions is used to produce a ball screw, stress is concentrated onto the edge portion, an adverse effect is given on the precision in shape of threaded groove in the ball screw.
Moreover, when a through-rolling die is used, the magnitude of angle xcex8 of inclination of the biting portion K gives an adverse effect on the precision in threaded groove in the ball screw shaft. In order to improve the precision, the angle xcex8 of inclination of the rolling die may be reduced (excessive reduction of the angle xcex8 of inclination is not good). However, this requires a die having too long a width that adds to production cost. Further, rolling requires increased pressing force that requires a large-scale apparatus.
Accordingly, the present invention has been worked out paying attention to the unresolved problems of the related art. It is an object of the present invention to provide a thread rolling die which can form a threaded groove in a screw shaft by rolling at a reduced cost with a high precision. It is also an object of the present invention to provide a die producing process which can produce biting portion, straight portion and relief portion of the rolling die using the same die grinding wheel at a reduced cost.
A second object of the present invention is to provide a rolling die for ball screw which can form a high precision threaded groove on the periphery of a ball screw material by rolling.
A third object of the present invention is to provide a ball screw which can enhance the precision in positioning and reduce noise.
In order to accomplish the foregoing objects, a first aspect of the present invention is a rolling die for thread through-rolling comprising a dancette portion formed helically continuously on the periphery thereof, wherein the dancette portion becomes smaller away from a central portion to at least one end in a longitudinal direction thereof.
A second aspect of the present invention is a rolling die for ball screw comprising a straight portion formed on a central portion and having the same shaped dancette portion, a biting portion formed at one end of the straight portion in the longitudinal direction and having a helical protrusion, and a relief portion formed on the other end of the straight portion in the longitudinal direction and having a helical protrusion, wherein the helical protrusions formed on the biting portion and the relief portion each become continuously and gradually smaller away from the dancette portion on the straight portion.
A third aspect of the present invention is a process for the production of the rolling die for ball screw defined in the second aspect, comprising the steps of grinding forming the dancette portion on the straight portion of the die with uniform lead of working of a grinding wheel, forming one side of the dancette portion on each of the biting portion and the relief portion with the grinding wheel in such a manner that lead of working is changed to be greater than that at the straight portion as distancing away from the straight portion and continuously increases without changing the radial position of the grinding wheel and forming the other side of the dancette portion on each of the biting portion and the relief portion with the same grinding wheel in such a manner that lead of working is changed to be smaller than that at the straight portion as distancing away from the straight portion and continuously decreasing without changing the radial position of the grinding wheel; wherein the forming of the dancette portion on the straight portion, the biting portion and the relief portion is continuously performed.
A forth aspect of the present invention is the rolling die for ball screw comprising a straight portion formed on a central portion and having the same shaped dancette portion and a biting portion formed at one end of the straight portion in the longitudinal direction and having a helical protrusion, wherein the helical protrusion on the biting portion becomes continuously and gradually smaller away from the dancette portion on the straight portion.
A fifth aspect of the present invention is a ball screw having a ball threaded groove formed by rolling by the rolling die defined in the first, second and fifth aspects.